1. Field of the Invention
The present invention relates to a drive force transmission apparatus disposed between a driving shaft and a driven shaft in a four-wheel-drive vehicle in order to transmit torque therebetween.
2. Description of the Related Art
One type of drive force transmission apparatus is disclosed in Japanese Utility Model Application Laid-Open No. 5-12767. Such a drive force transmission apparatus comprises an inner rotatable member and an outer rotatable member, which are supported coaxially and in a relatively rotatable manner with each connected to either a driving shaft or a driven shaft, a frictional clutch provided between the inner and outer rotatable members in order to transmit torque therebetween through frictional engagement, and pressing-force generation means provided between the inner and outer rotatable members for generating a pressing force in accordance with a rotational speed difference between the driving and driven shafts and for transmitting the pressing force to the frictional clutch to thereby frictionally engage the frictional clutch.
In this type of drive force transmission apparatus, when relative rotation occurs between the driving and driven shafts due to the occurrence of a rotational speed difference therebetween, a pressing force is generated in the pressing-force generation means in accordance with a rotational speed difference between the shafts. The thus-generated pressing force is transmitted to the frictional clutch, causing the frictional clutch to become frictionally engaged. As a result, the transmission of torque is established between the driving and driven shafts in accordance with the rotational speed difference therebetween. Thus, the vehicle equipped with the drive force transmission apparatus enters standby four-wheel-drive (standby 4WD).
When this type of drive force transmission apparatus is employed in a four-wheel-drive vehicle, in addition to the transmission of a drive force in accordance with a rotational speed difference between the front and rear wheels during normal traveling, the following operations are preferably established. Under severe traveling conditions, such as an attempt to extract a wheel trapped in a gutter or the like, traveling on a rough road, traveling on sand, etc., the frictional clutch is engaged. Also, when control by the anti-lock brake system (ABS), vehicle stabilization control, or like control is performed, a drive force to be transmitted is made zero or very small, so that such control is performed smoothly while a disturbance is avoided.
Accordingly, when the torque transmission characteristics of a drive force transmission apparatus for use in a four-wheel-drive vehicle are fixed regardless of traveling conditions of the vehicle, the apparatus fails to satisfactorily cope with various traveling conditions of the vehicle as described above.
In the drive force transmission apparatus disclosed in the above publication, in order to smoothly perform control by ABS, vehicle stabilization control, or like control, there is employed means for controlling the volume of a fluid chamber which constitutes the pressing-force generation means and contains a viscous fluid. The volume of the fluid chamber is increased as needed to thereby limit the generation of a fluid pressure in the fluid chamber even when a rotational speed difference exists between the driving and driven shafts, so that the frictional clutch is restrained or prevented from being frictionally engaged by a pressing force induced by the fluid pressure.
The above-mentioned drive force transmission apparatus employs means for varying the volume of the fluid chamber through advancement/retreat of a plunger facing the fluid chamber. However, it is difficult to directly control the volume of the fluid chamber. Also, it is difficult for such means to engage the frictional clutch in order to cope with severe driving conditions, such as an attempt to extract a wheel trapped in a gutter or the like, traveling on a rough road, traveling on sand, etc.